BIOL2049 Antibiotics Lecture 4 PDF

BIOL2049 Pharmacology Semester 2 ANTIBIOTICS - Lecture 4 Inhibitors of Bacterial Protein...

BIOL2049 Pharmacology Semester 2 ANTIBIOTICS - Lecture 4 Inhibitors of Bacterial Protein Synthesis https://www.britannica.com/science/Streptomyces Dr. Fatima Pereira [email protected] Antibiotics: Mechanism of action  Learning objectives 1. Identify the reason why protein synthesis inhibitors are selective for bacteria 2. Identify the site of action of the following protein synthesis inhibitors: Chloramphenicol, Tetracyclines, Macrolides, Aminoglycosides 3. Describe mechanisms of resistance to aminoglycosides and chloramphenicol 4. List three serious side effects of protein synthesis inhibitors 5. List potential interactions between protein synthesis inhibitors and other drugs What makes protein synthesis inhibitors selective for bacteria Ribosome structure Prokaryotes versus Eukaryotes Prokaryotic (70S) 50S+30S Eukaryotic (80S) 60S+40S  Protein Synthesis Inhibitors that bind the 30S subunit Aminoglycosides : e.g. Streptomycin NH H2N OH Streptomyces griseus HO NH HOH2C O O H NH2 N HO HO CH3 O OH NH OHC First antibiotic to inhibit Gram negative organisms https://www.britannica.com/science/Streptomyces Broad Spectrum (also gentamicin or neomycin) New Jersey State microbe Saved millions of lives: Nobel Prize for Physiology or Medicine in 1952 Streptomycin: mechanism of action (1) Binds irreversibly to the 30S subunit on 16S rRNA 1. Freezes protein synthesis initiation Accumulation of 70S monosomes in a frozen initiation complex Important: it does not inhibit elongation. In hybrid cells sensitivity is dominant over resistance Streptomycin: mechanism of action (2) 2. Causes misreading of the genetic code destabilizes binding between the subunit and the “correct” tRNA simultaneously stabilize the binding of the subunit to the “wrong” tRNA, thereby effectively removing the discrimination between the correct and the wrong tRNA. Nobel Prize Ramakrishnan (2008) Biochem. Soc. Trans. 36, 567-574. Structure and function of the Ribosome Shown how different antibiotics bind to the ribosome  Streptomycin: Resistance and side effects Mechanisms of resistance to aminoglycosides: 1. Decrease uptake 2. Altered ribosomes (due to mutations where ) 3. Modification of Streptomycin Side effects: (which are not related to its action on protein synthesis) Muscle weakness – decreased release of acetyl choline (binding to Ca2+ sites on the presynaptic membrane) Reversible ototoxicity Nephrotoxicity Protein Synthesis Inhibitors that bind the 30S subunit Tetracyclines https://onlinelibrary.wiley.com/doi/full/10.1002/der2.49 Binds to the 16S rRNA Doxycycline Blocks binding of incoming aminoacyl-tRNA to the A-site Tetracycline: Site and mechanism of action https://onlinelibrary.wiley.com/doi/full/10.1002/der2.49 Tetracyclines interact with magnesium, calcium, etc  Tetracyclines: Uses and side effects Tetracyclines chelate calcium and prevents its absorption, therefore its use leads to calcium deficiency Tetracycline drugs such as doxycycline are used for skin infections (treatment of acne); respiratory infections, eye infections, anthrax Skin photosensitivity Narrow spectrum: Sarecycline Cutibacterium acnes  Protein Synthesis Inhibitors that bind the 50S subunit Chloramphenicol Bacteriostatic (reversible) Chloramphenicol Streptomyces venezuelae Binds 23S rRNA on 50S subunit Chloramphenicol: site and mechanism of action Inhibits peptidyl transferase: interferes with peptide bond formation Stops protein synthesis: accumulation of polysomes Problems with chloramphenicol use Resistance Chloramphenicol acetyl transferase Side effects Gray baby syndrome excreted by glucuronidation not developed in neonates Reversible anaemia – bone marrow depression Low incidence (about 1 in > 20,000) of aplastic anaemia, which can develop weeks after stopping the treatment – therefore only used for eye infections and life-threatening conditions Protein Synthesis Inhibitors that bind the 50S subunit Macrolides: Erythromycin Clarithromycin Better tissue penetration than Azithromycin Erythromycin Most potent against Gram positive bacteria Bacteriostatic (can be cidal in certain cases) Inhibit translocation (50S subunit) Used especially for those who cannot take penicillin (for respiratory tract infections) Macrolides: site and mechanism of action Bind near the peptidyl transferase Inhibit Transpeptidation/Translocation https://microbeonline.com/macrolides-action-resistance/  Macrolides: Resistance and side effects Producing organism is protected by dimethylation of a specific adenine its in 23S rRNA – A2058Ec Side effects: Clarithromycin inhibits the cytochrome P450 – especially CYP3A4: its use results in increased serum levels and potential increased toxicities of multiple drugs metabolized by these enzymes  Additional Literature BD Davis. Mechanism of bactericidal action of aminoglycosides. Microbiology and Molecular Biology Reviews, 1987 – Am Soc Microbiol. 341-350 Gualerzi, C.O., Brandi, L., Fabbretti, A. and Pon, C.L. (2013) Antibiotics: Targets, Mechanisms and Resistance. Wiley-VCH; 1st edition (5 Sept. 2013) Kohanski, M.A., Dwyer, D.J., and Collins, J.J. (2010) How antibiotics kill bacteria: from targets to networks. Nat Rev Microbiol 8: 423–435. DOI: 10.1038/nrmicro2333 Graber, E.M. (2021) Treating acne with the tetracycline class of antibiotics: A review. Dermatological Reviews 2: 321–330.